System to charge e-cars while driving

Engineers have demonstrated a practical way to use magnetism to transmit
electricity wirelessly to recharge electric cars, robots or even
drones. The technology could be scaled up to power electric cars as they
drive over highways, robots on factory floors and drones hovering over
rooftops.

Stanford engineers have taken a big step toward making it practical for
electric cars to recharge as they speed along futuristic highways built
to “refuel” vehicles wirelessly.

Although wireless charging pads already exist for smartphones, they
only work if the phone is sitting still. For cars, that would be just as
inconvenient as the current practice of plugging them in for an hour or
two at charging stations.

Three years ago, Stanford electrical engineer Shanhui Fan and Sid
Assawaworrarit, a graduate student in his lab, built the first system
that could wirelessly recharge objects in motion. However, the
technology was too inefficient to be useful outside the lab.

Now, in Nature Electronics,
the two engineers demonstrate a technology that could one day be scaled
up to power a car moving down the road. In the nearer term, the system
could soon make it practical to wirelessly recharge robots as they move
around in warehouses and on factory floors – eliminating downtime and
enabling robots to work almost around the clock.

“This is a significant step toward a practical and efficient system
for wirelessly re-charging automobiles and robots, even when they are
moving high speeds,” Fan said. “We would have to scale up the power to
recharge a moving car, but I don’t think that’s a serious roadblock. For
re-charging robots, we’re already within the range of practical
usefulness.”

Wireless chargers transmit electricity by creating a magnetic field
that oscillates at a frequency that creates a resonating vibration in
magnetic coils on the receiving device. The problem is that the resonant
frequency changes if the distance between the source and receiver
changes by even a small amount.

In their first breakthrough three years ago, the researchers
developed a wireless charger that could transmit electricity even as the
distance to the receiver changes. They did this by incorporating an
amplifier and feedback resistor that allowed the system to automatically
adjusts its operating frequency as the distance between the charger and
the moving object changed. But that initial system wasn’t efficient
enough to be practical. The amplifier uses so much electricity
internally to produce the required amplification effect that the system
only transmitted 10% of the power flowing through the system.

In their new paper, the researchers show how to boosts the system’s
wireless-transmission efficiency to 92%. The key, Assawaworrarit
explained, was to replace the original amplifier with a far more
efficient “switch mode” amplifier. Such amplifiers aren’t new but they
are finicky and will only produce high-efficiency amplification under
very precise conditions. It took years of tinkering, and additional
theoretical work, to design a circuit configuration that worked.

The new lab prototype can wirelessly transmit 10 watts of electricity
over a distance of two or three feet. Fan says there aren’t any
fundamental obstacles to scaling up a system to transmit the tens or
hundreds of kilowatts that a car would need. He says the system is more
than fast enough to re-supply a speeding automobile. The wireless
transmission takes only a few milliseconds – a tiny fraction of the time
it would take a car moving at 70 miles an hour to cross a four-foot
charging zone. The only limiting factor, Fan said, will be how fast the
car’s batteries can absorb all the power.

The wireless chargers shouldn’t pose a health risk, said
Assawaworrarit, because even ones that are powerful enough for cars
would produce magnetic fields that are well within established safety
guidelines. Indeed, the magnetic fields can transmit electricity through
people without them feeling a thing.

Though it could be many years before wireless chargers become
embedded in highways, the opportunities for robots and even aerial
drones are more immediate. It’s much less costly to embed chargers in
floors or on rooftops than on long stretches of highway. Imagine a
drone, says Fan, that could fly all day by swooping down occasionally
and hovering around a roof for quick charges.

Who knows? Maybe drones really could be practical for delivering pizza.